Gd-based Bulk Metallic Glasses Research Articles

Tuning magnetocaloric effect of Gd–Er–Al–Co metallic glass through crystallization

Magnetization and magnetocaloric effect (MCE) of as-cast Gd-based bulk metallic glass (BMG) Gd24Er32Co24Al20 and the crystallized samples were investigated. Reentrant-spin-glass-like behavior was observed in the as-cast sample. With increasing degree of crystallization, the magnetic frustration increased, which could be seen from the increased divergence of the zero-field-cooled and field-cooled magnetization behavior (i.e., the spin-glass-like freezing behavior). The Curie temperature and magnetization decreased after crystallization. The as-cast BMG showed a large maximum magnetic entropy change (− ΔSm) of 9.9 (J kg−1 K−1) under a field change of 5 T, which was comparable with that of Gd metal and other Gd-based metallic glasses. The maximum − ΔSm reduced to 6.7 (J kg−1 K−1) after crystallization, which was still superior to the Fe-/Co-based amorphous alloys. Their good MCE combining with high electrical resistivity, outstanding mechanical properties, tunable nature, and sufficiently soft magnetic property makes them to be attractive candidate for magnetic refrigerants in the low-temperature range.

Structural and magnetic properties of the Gd-based bulk metallic glassesGdFe2, GdCo2, andGdNi2from first principles

A structural and magnetic characterization of Gd-based bulk metallic glasses, ${\mathrm{GdFe}}_{2}, {\mathrm{GdCo}}_{2}$, and ${\mathrm{GdNi}}_{2}$, was performed. Models for the amorphous structures for two magnetic configurations, ferromagnetic and ferrimagnetic, were obtained by means of a first-principles-based method, the stochastic quenching. In all three cases, the ferrimagnetic configuration was energetically more stable than the ferromagnetic one, in perfect agreement with experiments. In the structural analysis, radial and angle distribution functions as well as calculations of bond lengths and average coordination numbers were included. Structural properties are in good agreement with experiments and do not depend on the magnetic configuration. The distribution of magnetic moments shows that amorphous ${\mathrm{GdFe}}_{2}$ and ${\mathrm{GdCo}}_{2}$ are both ferrimagnets, with antiparallel alignment of the magnetic moments of the two magnetic sublattices, whereas Ni nearly loses its magnetic moment in amorphous ${\mathrm{GdNi}}_{2}$, similar to the situation in its crystalline counterpart.

Achieving a large adiabatic temperature rise of Gd55Co25Al20 bulk metallic glass by minor Zn addition

Micro-alloying is a useful way to improve the magneto-caloric effect of Gd-based bulk metallic glasses (BMGs). However, the minor addition of other elements usually decreases the Curie temperature of BMGs. In this work, we fabricated Gd55Co25Al(20−x)Zn x (x = 2, 5) BMGs and studied their magnetic properties. It was found that a minor addition of Zn not only improved the peak value of the magnetic entropy change but also enhanced the Curie temperature of the fabricated BMG. As a result, the Gd55Co25Al15Zn5 BMG exhibits a rather high adiabatic temperature rise up to 5.1 K at a temperature of around 130 K under a field of 5 T.

Pressure-induced polyamorphic transitions in ytterbium-based bulk metallic glasses

The structure of Yb60Ca2.5Zn20Mg17.5 bulk metallic glasses was investigated by in situ angle-dispersive X-ray diffraction with a synchrotron radiation source. The glassy nature of the samples was stable up to ~30GPa at room temperature. The volume and bulk modulus as a function of hydrostatic pressure were extracted from the diffraction data. A sudden change was observed in the volume and bulk modulus during compression. Results indicated that pressure induced polyamorphic phase transition in Yb-based bulk metallic glasses compared with Pr- and Gd-based bulk metallic glasses. The critical pressure of the transition was about 10GPa. The delocalization of 4f electrons under high pressures was responsible for the amorphous-to-amorphous phase transition in Yb-based bulk metallic glasses.

Kinetics of crystallization process of Mg–Cu–Gd based bulk metallic glasses

The crystallization behavior of Mg61Cu28Gd11 and (Mg61Cu28Gd11)98Cd2 bulk metallic glasses was studied using DSC in the mode of continuous and isothermal heating, and its crystallization process and microstructure were confirmed by XRD and TEM. In continuous heating, the activation energies of glass transition, onset and peak crystallization were determined by the Kissinger method, which yields 110±12, 77±9 and 79±10kJ/mol, respectively, for Mg61Cu28Gd11 glassy alloy, and 144±10, 126±6 and 131±5kJ/mol, respectively, for (Mg61Cu28Gd11)98Cd2 glassy alloy. The isothermal kinetics was modeled by the Johnson-Mehl-Avrami equation. The Avrami exponent of the base alloy was in the range from 1.98 to 2.56 (±0.01), which indicated a decreasing nucleation rate and a diffusion-controlled growth. For Cd-added glassy alloy, the Avrami exponent was in the range from 3.26 to 4.08, which indicated an increasing nucleation rate. The activation energies in isothermal process were calculated to be 88±2 and 132±2kJ/mol, respectively, for the base and Cd-added glassy alloys. It was found that Mg2Cu phase was the primary phase in the initial crystallization and the strong affinity between Cd and Mg/Gd tended to impose resistance to the formation of Mg2Cu phase and thus improves the thermal stability.

The magnetocaloric effect of the Gd-based amorphous composite with Gd nanocrystals

The Gd-based amorphous/nanocrystal composite is prepared by controlling the cooling rate and the element ratio. The X-ray diffraction, differential scanning calorimeter and atomic force microscope/magnetic force microscope are used to confirm the composite microstructures from different perspectives. The magnetic test shows the great enhancement of magnetocaloric effect in the metallic glassy composite. The large magnetic refrigerant capacity (RC) up to 103 J. kg-1 is more than double the RC values of the Gd-based bulk metallic glass and pure Gd. The full width at half maximum of the magnetic entropy change (Sm) peak almost spreads over the whole low-temperature range, which is five times wider than that of the pure Gd. The maximum Sm approaches a nearly constant value in a wide temperature span (over 80 K). The super paramagnetic nanoclusters of the composite increase the magnetic refrigerant capacity greatly. In combination with the low magnetic hysteresis and large resistance, the metallic glass composite may be a potential candidate for the ideal Ericsson-cycle magnetic refrigeration.

Kinetic study on the non-isothermal crystallization of Gd 53Al 24Co 20Zr 3 bulk metallic glass

Gd-based bulk metallic glass has drawn strong attention because of its large magnetic entropy changes. Thermal stability of metallic glass is a very important issue for its application. In the paper, crystallization behavior of Gd 53Al 24Co 20Zr 3 bulk metallic glass was investigated using non-isothermal differential scanning calorimetric (DSC) technique. Attention was given to the analytic details. The crystallized volume fractions as a function of temperature were derived from the DSC signals, where heat capacity change between amorphous phase and crystalline phase was considered. The local activation energies at different crystallized volume fraction were estimated using Doyle-Ozawa and Agrawal methods. The results suggested that the Doyle-Ozawa equation was appropriate to get local activation energy due to its simplicity and accuracy. The local activation energy depended on the crystallized volume fraction. Function reflecting crystallization mechanism was also deduced. The crystallization mechanism of the Gd-based bulk metallic glass was discussed.

Structures and magnetocaloric effects of Gd65−xRExFe20Al15 (x = 0–20; RE=Tb, Dy, Ho, and Er) ribbons

Gd-based Gd65−xRExFe20Al15 (x = 0–20, RE = Tb, Dy, Ho, and Er) alloys in ribbon form have been prepared by rapid quenching technology. It is found that the Gd65−xTbxFe20Al15 ribbons are in amorphous state. However, for the Gd65−xRExFe20Al15(RE = Dy, Ho, and Er) ribbons, in addition to a diffuse diffraction band, peaks from hexagonal rare-earth-rich phases are present indicating a mixture of amorphous and crystalline phases. This indicates that the heavier rare-earth elements (Dy, Ho, and Er) will decrease glass formability in the Gd-based Gd65−xRExFe20Al15 (RE = Dy, Ho, and Er) ribbons. The magnetic entropy changes of these ribbons increase with increasing the contents of the heavier rare-earth elements. The maximal magnetic entropy changes of the Gd45RE20Fe20Al15 (RE = Tb, Dy, Ho, and Er) ribbons are located in the range of 4.46–5.57 J/kg K under 50 kOe. Refrigerant capacity values are obtained to be about 580–720 J/kg under 50 kOe. These values are comparable to or even higher than those of Gd-based bulk metallic glass alloys.

Enhancement of the glass-forming ability by Zr microalloying and its influence on the magnetocaloric properties of bulk amorphous Gd–Co–Al

Microalloying with 1 at. % zirconium dramatically increases the critical size from 3 to 8 mm for Gd-based bulk metallic glasses (BMGs). The enhancement of glass-forming ability originates from the increase in the packing efficiency and high compound forming tendency between Zr and major constituent elements. The maximum magnetic entropy change (−ΔSMmax) and relative cooling power (RCP) of Gd52.5Co18.5Al29−xZrx (x=1) BMG are 9.6 J/kg K and 8.9×102 J/kg, respectively. Heat capacity data demonstrate that the broad magnetic transition arising from topological disorder is responsible for the high RCP of Gd-based amorphous magnetocaloric materials.

Large magnetocaloric effect and enhanced magnetic refrigeration in ternary Gd-based bulk metallic glasses

The magnetocaloric effect and refrigeration capacity (RC) of Gd55Co20Al25 and Gd55Ni25Al20 bulk metallic glasses (BMGs) have been investigated. Large magnetic entropy changes ΔSM of 11.2 and 10.8 J kg−1 K−1 and large RC values of 846 and 920 J kg−1 are obtained for Gd55Co20Al25 and Gd55Ni25Al20, respectively, at a field change of 7 T. The RC value (640 J kg−1 at 5 T or 920 J kg−1 at 7 T) of Gd55Ni25Al20 BMG is larger than that reported for all magnetocaloric materials, including crystalline and amorphous materials measured under the same conditions. The large RC value is due to the broad ΔSM peak (more than 100 K), which is caused by the disordered structure of an amorphous material. The large ΔSM and RC values make these Gd-based ternary BMGs attractive candidates for magnetic refrigeration applications.

Correlation between the fragility of supercooled liquids and thermal expansion in the glassystate for Gd-based glass-forming alloys

Dilatometric measurements were performed to obtain the average thermal expansioncoefficients of a series of Gd-based bulk metallic glasses. The fragilities of these alloys weredetermined based on differential scanning calorimetry measurements. It was found thatthere is a linear correlation between the fragility parameter of supercooled liquids and theaverage thermal expansion coefficient in Gd-based bulk metallic glass-forming alloys.

Magnetocaloric effect in Gd-based bulk metallic glasses

Magnetocaloric effect and refrigerant capacity of Gd-based Gd53Al24Co20Zr3 and Gd33Er22Al25Co20 bulk metallic glasses are investigated. It is found that the magnetic entropy changes compare favorably with that of Gd and are slightly larger than that of the known crystalline magnetic refrigerant compound Gd5Si2Ge1.9Fe0.1. Their good refrigerant efficiency combining with high electrical resistivity, high thermal stability, outstanding mechanical properties, tunable nature, and sufficiently soft magnetic property make them an attractive candidate for magnetic refrigerants in the temperature range of 10–100K.

A bulk metallic glass based on heavy rare earth gadolinium

The rare earth Gd based Gd 60Al 10Ni 10Cu 20 bulk metallic glass (BMG) has been fabricated by Cu-mold casting method. The BMG-forming alloy exhibits a glass transition, obvious multi-crystallization, near eutectic melting and paramagnetic property. However, the alloy does not have a high reduced glass transition temperature ( T rg = 0.55) which is a common feature for other known BMGs. We also find that a series of alloys with identical composition of M 60Cu 20Ni 10Al 10 with different bases (M = Zr, La, Nd, Pr, or Ce) can be easily cast into bulk glass. The Gd-based BMGs could have potential applications as functional materials.